Drifted snow is a meteorological element constituting precipitation along with rainfall, and has been utilized as very important data for the planning on water utilization and flood control as well as meteorological analysis. In particular, the drifted snow has very high importance as a basic material in forecasting and analyzing snow melting that covers the greater part of a river outflow and a groundwater outflow.
There are various kinds of a snow-depth meter for measuring the drifted snow, such as ultrasonic waves, laser beams, images and the like. According to the principle of observation, a method of measuring the drifted snow is divided into an area measuring method using the amount of time that it takes the ultraviolet waves to be reflected to a snow surface and to return, a point measuring method using a light wave and a phase difference by irradiating laser beams, an image measuring method using an outside camera.
Meanwhile, according to guidelines of the World Meteorological Organization (WMO, 2008), the current level of snowdrift observation devices and technologies throughout the world is not superior to the accuracy of eye observation which is directly observed by a person.
The reason is because drifted snow is influenced by the wind, and thus a curvature is generated on a snow surface, or snow covered on a snow measuring plate is moved in any direction and thus it is difficult to obtain an average value. Also, the drifted snow falls down or is compacted due to the weight of snow, and freezes and melts repeatedly according to a temperature. This has a direct effect on the soil on which the snow measuring plate is installed, thereby causing rising and falling of the soil. Thus, due to the rising and falling of the soil, the standard point observed in the sensor is changed, and thus, it is difficult to obtain accurate observation data, and an error in observation resulting from it is frequently generated.
With regard to auto snow measuring apparatuses which have been currently operated at Korean meteorological observatories, an ultrasonic snow measuring apparatus has been installed at 72 observatories, and drift snow has been observed using CCTV (closed-circuit television), a snow-depth meter by laser and the like together. However, according to each of the observatories, the soil states of an observation field in which the snow measuring plate is installed are different from each other, and thus it is difficult to obtain accurate observation data because a standard point measured in the sensor is changed due to rising and falling of the soil of the lower part of the snow measuring plate generated when snow melts and is absorbed into the soil.
As illustrated in FIG. 1, an ultrasonic snow meter which has been currently used in the Meteorological Administration is configured such that an instrument tower 10 in which a sensor for observing drifted snow is installed, and a snow measuring plate 20 are separated from each other.
Since the instrument tower 10 is erected by laying concrete in a lower part of the soil, it is not almost influenced by rising and falling of the soil due to drifted snow or precipitation. On the contrary, the snow measuring plate 20 is installed by putting it on the ground directly or designating a certain section on the ground. This method is largely influenced by drifted snow and precipitation. That is, the soil of a lower part of the snow measuring plate 20 rises and falls repeatedly from several mm to several cm depending on a state of the soil when drifted snow melts and water flows into the soil. Accordingly, since the standard point observed in the sensor installed in the instrument tower 10 is changed, it is problematic in that it is difficult to obtain accurate observation data, and an error in observation resulting from it is frequently generated.
Meanwhile, FIG. 2 is a graph showing a height variation of a snow measuring plate generated when water flows into the soil based on observation data of a general snow-depth meter by laser. Specifically, a water screen was formed on the snow measuring plate due to light rain between 4:00 to 7:00 a.m. on Jun. 30, 2010, and accordingly, the error of an increase in a value observed by the snow-depth meter by laser was generated. However, from 7:00 a.m. when the rain stopped, a correction value was reduced to ‘0’ again. Since then, from 7:00 am., a temperature increased, and thus, due to the water absorbed in the soil, the correction value was reduced from ‘0’ to about ‘−2.5 mm.’
Furthermore, on Nov. 26, 2009, a temperature increased after the inflow of dense fogs from a thin mist from 3:00 a.m. to 11:00 a.m., and thus a correction value was reduced from ‘0’ to about ‘−8 mm,’ and since a value observed by an ultrasonic snow-depth meter is not expressed as a minus value, a variation in the snow measuring plate could not be confirmed.
Analyzing such a case, the soil influenced by precipitation or dense fogs causes rising of the soil due to water. This has an influence on a lower part of the soil on which the snow measuring plate is located, and a standard value, thereby causing an error in value of drifted snow observed practically.